(1) Field of the Invention
The present invention relates to the field of guiding projectiles. The invention relates more particularly to guiding projectiles with the assistance of a laser beam.
The present invention relates both to a method and also to a device for assisting aiming at a target. The present invention also relates both to a method of guiding a projectile by means of a laser beam and using such a method of assisting aiming, and also to a device for guiding a projectile by means of a laser beam and fitted with such a device for assisting aiming.
(2) Description of Related Art
Laser beam guidance is used in particular by the military for guiding a missile or any other projectile to a target illuminated by means of a laser beam. This technique is known as “semi-active laser homing” or by the initials SALH. In this technique, and as described in document U.S. Pat. No. 4,143,835, a laser beam is kept pointed on a target by an operator, often referred to by the term “shooter”. Reflections of the laser beam are then dispersed in a multitude of directions by reflection on the target. A projectile, such as a missile, can then be launched or released towards the target.
When the projectile is close enough to the target, a receiver device included in the projectile receives a portion of the laser beam reflected by the target, and then determines the source of this reflected portion of the laser beam, i.e. the target. The trajectory of the projectile is then adjusted towards the source. The projectile, which properly speaking does not possess any autonomous means for detecting the target, is then guided solely towards the source by the portion of the reflected laser beam that it receives.
Thus, so long as the laser beam is kept pointed on the target and so long as the guidance device of the projectile receives a portion of the reflected laser beam, the trajectory of the projectile can be corrected so as to guide the projectile exactly onto the target. Emission of the laser beam is thus dissociated from the projectile and may be performed by an operator, for example. In contrast, the operator needs to have the target in view in order to be able to point the laser beam at it. Advantageously, the zone from which the projectile is launched is totally independent of the zone from which the laser beam is emitted. The laser beam is emitted by a laser-beam generator such as a laser designator.
In order to guide the projectile accurately towards the target, it is essential for the laser beam illuminating the target to be of good quality, which depends essentially on the percentage of the laser beam that actually reaches the target. A laser beam used for guiding a projectile is generally constituted by a succession of pulses emitted at time intervals that may be regular or irregular, but by that in any event are known in order to be identifiable by the receiver means of the projectile.
Consequently, all of the laser beam pulses that are not pointed at the target will guide the projectile away from the target, whereas all of the laser beam pulses that are pointed at the target will guide the projectile exactly to the target.
A laser beam used for guiding a projectile could equally well be a continuous laser beam.
The aiming procedure always begins by scanning the environment that is visible to the operator looking for targets, and continues by ceasing to scan in order to focus on a target. Under such circumstances, the operator needs to point the laser beam permanently at the target in order to guide the projectile.
Nevertheless, it can be difficult to point accurately at a target permanently, in particular when the target is moving relative to the operator. Specifically, the target may be a moving vehicle, e.g. a land vehicle or an aircraft. The operator may also be moving, e.g. being on board a land vehicle or an aircraft.
Furthermore, the operator does not have direct visual feedback about the point in the environment that is actually illuminated by the laser beam. Specifically, the reflection of the laser on the target is generally not visible to the operator. The operator thus can only rely on the aim, e.g. performed using crosshairs of telescopic sights for a portable laser designator, or else by using display means incorporated in a helmet for a laser designator on board a vehicle. Consequently, there may be an offset between the crosshairs and the laser beam proper, giving rise to an aiming error that is not perceived by the operator. Only the impact of the projectile informs the operator about the accuracy of the initial aim and the aiming error, if any. Specifically, when the projectile has missed the target, the operator might possibly correct the aim as a function of the point of impact of the projectile relative to the target, but only after a first miss.
Furthermore, it should be recalled that a laser beam is a particular kind of light beam made up of light that is coherent and concentrated. The term “laser” is an acronym for “light amplification by stimulated emission of radiation”. Also, the term “light beam” is generally used to designate a beam made up of light that is visible to the human eye. Nevertheless, and by extension, it is also possible to use the term “light beam” to designate a light beam made up of electromagnetic waves that are not visible, e.g. situated in the infrared or the ultraviolet ranges. A laser beam may thus equally well be a light beam that is situated in the range visible to the human eye or in a range that is not visible.
Furthermore, a device for correcting projectile trajectories is described in document FR 2 719 659. That corrector device emits a guide beam for guiding projectiles that is directed towards a target. The guide beam is subdivided into at least five partial beams, a central partial beam that is indeed directed on the target, and at least four partial beams that are inclined relative to the central partial beam. The projectiles illuminated by an inclined partial beam are therefore not directed towards the target and they have their trajectories corrected accordingly.
Furthermore, documents US 2009/078817 and EP 2 642 238 are known, which describe a system for guiding a missile or any other projectile onto a target illuminated by means of a laser beam.
In particular, document US 2009/078817 describes a projectile guidance system seeking to reduce the number of pulses of the guide beam in order to reduce the total energy sent towards the target. That device requires communication between the projectile and the guide-beam generator in order to synchronize reception of the reflected guide beam and emission of the guide beam.
Also known is document U.S. Pat. No. 6,069,656, which describes a method and a device for stabilizing images for use in a system for providing a projectile with laser guidance. In particular, that method makes it possible to display the total scene picked up by a camera, or else only a portion of the scene corresponding to a zoom mode. Furthermore, that method makes it possible to display a single point of contact of the guide beam, with the distance between crosshairs situated on the target and the point of contact being used in order to correct the guide beam.
Finally, the following documents US 2013/087684, WO 2016/009440, and U.S. Pat. No. 6,023,322 are known, which describe a system and/or a method for analyzing quality criteria of a guide beam in a system for laser designation of a target.
According to document US 2013/087684, image capture means enable the guide beam to be analyzed by means of points of contact of the guide beam on the target.
According to document WO 2016/009440, radiation capture means serve to analyze the guide beam as reflected by the target, and in particular the arrival time of the guide beam as reflected by the target onto the sensor, its angle of arrival, and/or its arrival position on the sensor.
Document U.S. Pat. No. 6,023,322 serves to determine the ratio between the number of contact points of the guide beam as reflected by the target and the number of pulses of the emitted guide beam, e.g. making it possible to find the best zone of the target on which to aim the guide beam.